1885.] on hoio Thought presents itself in Nature. 189 



within a box, and by this very coarse internal motion have done 

 enough to alter the inertia of the system, i. e. the way it behaves 

 under impressed forces ; and that not merely as respects its quantity 

 but its kind. By an equally rough contrivance we can create by 

 motions a property resembling elasticity ; and so in other cases. 



Conclusions — I. The External Source of Sensation is Motion. 

 The general outcome of all such inquiries is that we are con- 

 fronted with the fact revealed to us by science, that every phenomenon 

 of the outer world which we can perceive by any of our souses, is 

 simply a mass of motions. The z of our inquiry is entirely motion. 



II. So ALSO IS WHAT OCCURS WITHIN OUR BoDIES. 



So also is the y, which means that part of the phenomenon which 

 although still outside the nerves, is in more immediate relation to 

 them. Nerves are not directly acted upon by what goes on outside 

 our organs of sense, but by the change within our organs of sense 

 which the external events occasion. For example, the molecular 

 motions that are going on in the object we look at, do not reach 

 our optic nerve. What they do is to produce luminous undulation 

 around that body. Nor is it even this undulation, the light, that 

 acts on our optic nerve. What it does is to make a change in the 

 black pigment of the eye, and it is the new chemical substances thus 

 evolved which are what act upon an apparatus of rods, cones, and 

 bulbs in such a way that the latter become able to operate on the 

 nerve. The new series of events that occur within the eye would, if 

 fully understood, constitute the y part of the scientific naturalist's 

 explanation ; and although the details are imperfectly understood, 

 they are all of those kinds that consist of changes of motion. 



So again of the next, or x part of the scientific exi^lanation. This 

 is what occui's within the nerve fibres while they are receivino- the 

 impression from without, and exercising the j^ower to act upon the 

 brain which that stimulus has conferred upon them. Much is not 

 yet known about what occurs within the nerve, but in consequence of 

 the processes that go on, the nerve wastes and requires nourishment ; 

 it undergoes change, and the change which it undergoes advances 

 inwards towards the brain at a rate that can be measured, and which 

 is about the speed of the fastest railway train, a moderate sjDced com- 

 pared with many others we meet with in nature. Here all that has 

 been made out betokens that what occurs within the nerves is entirely 

 made up of motions. 



And, finally, the same is true of the brain itself. The motions 

 that go on within that most wonderful organ are probably the most 

 intricate that are known anywhere to prevail. But tliou<Th un- 

 fortunately very little is understood about the operations that are 

 going forward, there is nothing to raise the most distant suspicion 

 that if an adequate examination could be made, the scientific naturalist 



